Exhaust gas recirculation (EGR) systems were introduced in the early '70s to reduce an exhaust emission that was not being cleaned by the other smog controls. Nitrogen oxide and nitrogen dioxide (both commonly referred to as “NOx”) are formed when temperatures in the combustion chamber get too hot. At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons and the presence of sunlight, produces an ugly haze in our skies known commonly as smog.
In a typical automotive engine, EGR is used as a technique to reduce the amount of NOx formed during the internal combustion process. EGR involves the recirculation of a portion of an engine's inert exhaust gas back to the engine's cylinders to dilute the incoming air mix with the inert exhaust gas. This process lowers the adiabatic flame temperature, increases the specific heat capacity, and in the case of diesel engines, reduces the amount of excess oxygen of the incoming air mix. Because NOx forms faster at higher temperatures, the combination of increased heat capacity and lower combustion temperature reduces the amount of NOx formed.
Combustion engines perform work through combusting hydrocarbons to create a pressure pulse generating a pressure differential across the engine, and further converting that pressure into mechanical work. Maintaining this pressure differential is essential to the efficient functioning of the engine, and therefore the introduction of backpressure into the engine is undesirable. However, many internal combustion engines use a portion of the generated pressure difference to operate an EGR system, blending exhaust gas with intake air. As lower emissions are targeted and the demand for fuel efficiency and power density of combustion engines continues, many designers of internal combustion engines are challenged to improve the management of pressure within the engine.
In order for EGR to flow into the intake manifold, exhaust gas pressures must be higher than intake gas pressures. Traditionally, this requires that the exhaust manifold pressure be maintained higher than the intake manifold pressure. The requirement for higher exhaust manifold pressure is undesirable, as it creates extra backpressure on the engine. As such, the engine pistons need to work harder to push the exhaust out, which reduces the work that reaches the crankshaft. Accordingly, the use of EGR compromises the efficiency of the engine.
The control of EGR flow rates is typically achieved by the use of controlled backpressure using a turbocharger, often a variable geometry turbocharger (VGT). The VGT must control the desired work to compress inlet air and the desired exhaust manifold pressure to control the EGR flow rate. As a result, the control of the VGT is complex.
Typical heavy duty engines run about 15% to 30% EGR, depending on the operating condition of the engine and the type of after treatment system used. In most heavy duty engines, the exhaust manifold is common between all of the cylinders, and a pipe connects the exhaust manifold to a control valve, an EGR cooler, and then to the intake manifold. Thus, to vary the amount of EGR run (to maximize engine efficiency and minimize NOx emissions), complex sensor and control systems must be used to measure certain system aspects and control the valve, the VGT, the after treatment system, etc. This complex EGR system increases manufacturing complexities and costs, which can also lead to warranty issues.
Thus, it can be appreciated that there is a need for a lower cost, simplified EGR system and components that reduce backpressure on the engine and improve engine efficiency.